1. Field of the Invention
This invention relates to an improved process for making sputtered platinum/cobalt multilayer films and to the films made by this method.
2. References
Thin films with perpendicular magnetic anisotropy are potential candidates in high density magnetic and magneto-optical recording. Examples of materials useful in preparing such films include the oxide garnets and ferrites, amorphous rare-earth transition metal alloys, metal alloys such as CoCr, and metal multilayers of Pt/Co and Pd/Co, as disclosed in P. F. Carcia, U.S. Pat. No. 4,587,176.
In order to be useful for magneto-optical recording, a material must have additional attributes besides perpendicular magnetic anisotropy. These requirements include a square hysteresis loop, a sufficient Kerr effect, a large room temperature coercivity H.sub.c, and switching characteristics compatible with available laser power and magnetic field strength. All these necessary properties are exhibited by evaporated Pt/Co and Pd/Co multilayers as described in recent publications, see, e.g., W. B. Zeper et al., J. Appl. Phys. 65, 4971 (1989) and F. J. A. M. Greidanus, Appl. Phys. Lett. 54, 2481 (1989).
Pt/Co multilayers are preferred over Pd/Co for magneto-optical recording because they exhibit a higher Kerr effect. Sputtering is the preferred manufacturing process for preparing these multilayer films because it is simpler than other processes and the results obtained are more reproducible. However, sputtered Pt/Co multilayers have coercivities too small for magneto-optical recording. For example, Ochiai et al., Jap. J. Appl. Phys. 28, L659 (1989) and Ochiai et al., Digest of the Int'l. Mag. Conf. -1989. Wash., D. C., report H.sub.c of only 100-350 Oe (8-28 kA/m) for sputtered Pt/Co multilayers which they prepared by using argon as the sputter gas. These values of H.sub.c are less than or the same order of magnitude as that used as a write-magnetic field, which is typically about 40 kA/m. As a result, write-magnetic fields can change nearby previously written information in known Pt/Co multilayers when writing new information.
In contrast, evaporated Pt/Co multilayers have H.sub.c of about 1000 Oe (80 kA/m), which is sufficiently high to preserve adjacent, previously-written information during a writing process.
Y. Ochiai et al., EP 0304873 disclose studies of sputtered Pt/Co multilayers including the use of underlayers to increase H.sub.c. However, generally only marginal improvements in H.sub.c were achieved and the singular best result of 700 Oe (56 kA/m) requires a 1000 A (100 nm) thick Pt underlayer. This is impractical for most magneto-optical recording applications because it prohibits reading and writing information from the substrate side and the large heat capacity and thermal diffusivity of such a thick Pt layer will likely prevent writing with the limited power available with current solid-state lasers.
All known references to sputtered Pt/Co multilayers, and to sputtered metal multilayers in general, disclose the use of argon as the sputter gas.
The goal of the instant invention is to provide a process for directly sputtering Pt/Co multilayers with high coercivities and suitable for magneto-optical recording.